The present invention relates to gas turbines having rotational components cooled by a thermal medium flowing within the gas turbine rotor and particularly relates to coatings on the thermal medium supply and return tubes, supporting elements and seal assemblies therefor to reduce the coefficient of friction between the tubes and the supporting elements and between seal surfaces. In my prior U.S. patent application Ser. No. 09/334,187, filed Jun. 16, 1999, there is disclosed a gas turbine having a closed cooling circuit for supplying a thermal medium, for example, cooling steam from a rotor bore, radially outwardly toward the rim of the rotor, axially along the rotor rim and radially outwardly to turbine buckets for cooling the buckets. Additionally, the cooling circuit includes a return for the now-heated cooling thermal medium to a supply via the rotor rim, and the bore tube assembly. In that arrangement, a plurality of circumferentially spaced supply and return tubes extend through openings in stacked rotor wheels and spacers forming the rotor body. The supply and return tubes have lands at axially spaced positions along the tubes for contact with support elements in the wheels and spacers.
These axial tubes and support elements are located at a significant radial distance from the axis of the rotor. Those locations subject the tubes to substantial centrifugal forces during turbine operation, which causes significant contact forces between the tubes and the supporting elements. Moreover, because the tubes carry the thermal cooling medium, the tubes will heat more quickly than the rotor structure, causing axial displacement of the tubes relative to the rotor structure and supporting structure. The contact force caused by the centrifugal action of the rotor rotation, coupled with the contact friction coefficient, produces a substantial axial resistance to thermal expansion of the tubes. The magnitude of this resistance to axial displacement of the tubes is a direct determinant of the state of axial stresses in the tubes. Consequently, there is a need to reduce the axial stresses of the thermal medium carrying tubes to improve the service life of the tubes.
In addition, there are seal assemblies at the interface of axially adjacent wheels and spacers to seal between the respective cavities between the wheels and spacers surrounding the tubes and cavities outwardly of the tube supporting elements. Those tube supporting elements each include a bushing, an axially opposite sleeve spaced from the bushing and an annular tapered seal ring in an annular recess in the bushing engaging the bushing and the sleeve. The mating surfaces of the sealing ring and the bushing and sleeve also wear over time, requiring replacement of the seal and ring.
In accordance with a preferred embodiment of the present invention, to reduce the axial stresses in the thermal medium carrying tubes, the coefficient of friction between the tubes and their supporting elements is substantially reduced. Particularly, coatings are provided on one or both of the contacting surfaces between the tubes and the supporting elements for the tubes. Particularly, the coatings are provided on lands of the tubes and the inside contact surfaces of bushings supporting the tubes in the rotor. The surfaces to be coated are first roughened, for example, by shot peening to impart compressive stresses in the metal. A cobalt-based alloy, for example, Triballoy T800, is applied to the roughened surface at high temperature. The coating is sprayed onto the surface at high velocity to form a substantial bond at the molecular level with the nickel-based alloy of the tubes and support elements, e.g., bushings, and without disruption of the parent metal. This initial coating is then machined to provide a smooth base for the application of a second coating and to provide a smooth interface between the tubes and supporting elements. The second coating is an anti-wear/fretting/galling ceramic-based lubricant. This lubricant is sprayed onto the initial coating and baked, for example, at 1000xc2x0 F., for about one-half hour and at reduced temperature, for example, 500xc2x0 F., for about another half-hour. The lubricant is a dry film lubricant containing molybdenum disulfide, graphite, a lead-based glass frit and antimony trioxide. The lubricant contains less than 1% of water. With the foregoing coatings applied both to the outer surface of the tube lands and to the inside arcuate surfaces of the bushings, a very low coefficient of frictional contact between those surfaces is provided, affording low resistance to axial movement of the tubes relative to the supporting elements. This, in turn, improves the wear life of the interfacing parts.
The coatings are also applied to the contacting surfaces of the seal assemblies and particularly the seal ring and the adjacent bushing and sleeve. Thus, the annular recess formed in the bushing and the axial end face of the sleeve are coated, as previously described. Additionally, the inner and outer contact surfaces of the sealing ring are likewise coated such that a low coefficient of friction contact between the sealing ring and the bushing and sleeve is afforded.
In a preferred embodiment according to the present invention, there is provided in a gas turbine having metal tubes for conveying a cooling medium and metal support surfaces for the tubes, a coating on one of the tubes and surfaces affording a low coefficient of friction between one of the tubes and the surfaces at an interface therebetween, comprising a first coating of cobalt-based alloy overlying base material of one of the tubes and the surfaces and a second ceramic-based coating overlying the first coating.
In a further preferred embodiment according to the present invention, there is provided for a gas turbine, a tube for supplying a cooling medium, said tube having raised lands at axially spaced locations therealong for mounting the tubes in support elements for the turbine, a coating on the outer surfaces of the lands affording a low coefficient of friction between the lands and the supporting elements at an interface therebetween, the coating comprising a first cobalt-based alloy overlying the lands of the tube and a second ceramic-based coating overlying the first coating to reduce friction between the lands and the supporting elements.
In a still further preferred embodiment according to the present invention, there is provided in a gas turbine having a rotor comprised of axially stacked rotor wheels and spacers and aligned openings spaced from an axis of rotation of the rotor, the aligned openings of the wheels and spacers receiving a tube extending in an axial direction, a seal assembly about the tube at an interface of one of the wheels and one of the spacers, comprising a generally annular seal support in part received in a first opening in one of the wheels and spacers and including a generally radially extending flange in axial registration with an opposed face of another of the wheel and spacer, a seat bushing in part received in a second opening of another wheel and spacer in axial opposition to the first opening and having a seat, a seal between the seal support and the bushing, the seal including an annular frustoconically-shaped element having a radial outer edge portion engaging the seal support and a radially inner edge portion engaging the seat for sealing between the seal support and the seat, a first cobalt-based alloy overlying base material of one of the seal supports and the outer edge portion of the seal ring and a second ceramic-based coating overlying the first coating, a third cobalt-based alloy overlying one of the seat and the radially inner edge portion of the seal element and a fourth ceramic-based coating overlying the third coating.